with_name.exs

defmodule WithName do
  def with_name(name) do
    fn :get_and_update, data, next ->
      get_and_update(data, next, name, [], [])
    end
  end

  defp get_and_update([%{"name" => name}=map|t], next, given_name, get_acc, update_acc) when name == given_name do
    {get, update} = next.(map)
    get_and_update(t, next, given_name, [get|get_acc], [update|update_acc])
  end

  defp get_and_update([h|t], next, name, get_acc, update_acc) do
    get_and_update(t, next, name, get_acc, [h|update_acc])
  end

  defp get_and_update([], _next, _name, get_acc, update_acc) do
    {Enum.reverse(get_acc), Enum.reverse(update_acc)}
  end
end

x = %{"layers" => [%{"name" => "x"}, %{"name" => "y", "objects" => []}]}
IO.inspect update_in(x, ["layers", WithName.with_name("y"), "objects"], fn _ -> ["a"] end)